This invention relates to the conversion of energy from naturally occurring sources of mechanical energy, and particularly to the conversion of the mechanical energy present in ocean surface waves to useful energy.
In many known systems for capturing surface wave energy, a float is used for being vertically oscillated in response to passing waves. The float is rigidly coupled to an energy converter which is driven in response to vertical movements of the float. In one system, described in U.S. Pat. Nos. 4,773,221 and 4,277,690 (the subject matter of which is incorporated herein by reference), an open-ended hollow tube is rigidly suspended beneath a float, the tube being completely submerged and in vertical orientation.
The tube vertically oscillates in the water in correspondence with movements of the float and, in the absence of anything within the tube, the tube moves freely relative to the column of water within the open-ended tube. In one embodiment, a movable piston is disposed within the tube for blocking relative movements between the water column and the tube. As the tube and float oscillate within the water, the mass of water within the tube tends to block corresponding movements of the piston, hence the piston moves relative to the tube. Actual movement of the piston does occur, however, and provided the entire system is oscillating at it natural resonant frequency, relatively large amplitude oscillations of the piston can occur. The moving piston drives an energy converter fixedly mounted, e.g., within the float, for converting the piston movements to useful energy.
While these float driven tube systems work, efficient operation requires that the natural resonant frequency of the system closely matches the frequency of the ocean waves driving the system.
While this can be generally accomplished at a specific site and specific time, including means for adjusting the resonant frequency of the system in response to changing surface wave frequencies, a problem is that, at any instant, multiple poorly defined surface waves are present whereby much of the wave energy present can not be efficiently transferred to the oscillating system. Also, the means for adjusting the resonant frequency of the device generally involves changing the water mass within the device. Since this mass is quite large, it is not readily changed.
A feature of the present invention is that a relatively high efficiency of operation is obtained which is relatively insensitive to surface wave variations.